The recent discovery of a neuronal signature of consciousness by researchers from ULg’s and CHU Liège’s Coma Science Group, has helped to better understand the emergence of conscious states in human beings. In addition, this has led to a new test which, once it has been validated, should help to reliably establish the existence or nonexistence of residual consciousness in patients whose brain has suffered extensive damage.

What is consciousness? Owing to its many definitions, the polysemic usage of the term has long since obscured all debates concerning it. Nowadays, however, there seems to be a consensus on an operational definition: consciousness resides in the ability to relate mental states to oneself.

According to the work of the Coma Science Group, which is part of ULg’s and CHU Liège’s Cyclotron Research Center (CRC), and studies carried out by the laboratories of Stanislas Dehaene and Lionel Naccache (Institut du Cerveau et de la Moelle Epinière, Paris), multiple unconscious mental representations are continuously competing with each other on the edge of the “global conscious workspace” (terminology inspired by the Global Workspace Theory,

Relying mainly on functional neuroimaging techniques, the neurosciences are endeavouring to systematically identify the neuronal correlates of consciousness. In 1999, the group of Professor Steven Laureys, currently an FNRS senior research fellow and head of the Coma Science group, was able to observe brain activity in certain brain-damaged patients using positron emission tomography (PET) in two successive situations: first of all, when they were in a vegetative state and then when they had regained consciousness. By comparing these two states, researchers were able to identify a crucial frontoparietal network giving access to consciousness. Published in NeuroImage and Journal of Neurology, Neurosurgery & Psychiatry(1), these discoveries helped to determine the Global Workspace neuronal correlate and its “connectivity hypothesis” of consciousness.

In patients in a vegetative state, part of the brain is logically still active, but only in the primary areas (visual, auditory, somatosensory, etc.) and not in the hierarchically superior regions of the associative cortices of the frontoparietal network from which they are disconnected. In 1999, Steven Laureys and his team already sensed that the vegetative state was, above all, a disconnection syndrome of thalamo-cortical “loops” and between the primary areas and those in the conscious workspace. They further demonstrated this in an article published in the The Lancet(2) in 2000.

To put it plainly, consciousness involves the activation of neuronal connectivity distributed in the frontoparietal network constituting the conscious workspace(3), as well as corticocortical and thalamocortical loops, i.e. circuits that respectively form a “resonance” between regions of the cortex and between the thalamus and regions of the cortex. Consciousness seems to be indissociable from these feedback loops, which constitute the  brain’s « theater spotlight » which allows an unconscious mental representation to enter into the conscious workspace.

(1) Impaired effective cortical connectivity in vegetative state, Laureys S, Goldman S, Phillips C, Van Bogaert P, Aerts J, Luxen A, Franck G, Maquet PNeuroimage 9 (1999) 377-382 and Cerebral metabolism during vegetative state and after recovery to consciousness, Laureys S, Lemaire C, Maquet P, Phillips C, Franck G Journal of Neurology Neurosurgery and Psychiatry 67 (1999) 121
(2) Laureys S., Faymonville M.-E., Luxen A., Lamy M., Franck G., Restoration of thalamocortical connectivity after recovery from persistent vegetative state, in The Lancet 355 (2000) 1790-1791.
(3) More precisely, the cardinal elements of this central network are the prefrontal cortex, the anterior cingulate cortex, certain regions of the parietal and temporal cortices, as well as the thalamic nuclei.